The present invention relates generally to semiconductor laser device, and in particular to a semiconductor laser device used as a pumping source for an optical amplifier.
With the proliferation of multimedia features on the Internet in the recent years, there has arisen a demand for larger data transmission capacity for optical communication systems. Conventional optical communication systems transmitted data on a single optical fiber at a single wavelength of 1310 nm or 1550 nm, which have reduced light absorption properties for optical fibers. However, in order to increase the data transmission capacity of such single fiber systems, it was necessary to increase the number of optical fibers laid on a transmission route, which resulted in an undesirable increase in costs.
In view of this, there has recently been developed wavelength division multiplexing (WDM) optical communications systems such as the dense wavelength division multiplexing (DWDM) system wherein a plurality of optical signals of different wavelengths can be transmitted simultaneously through a single optical fiber. These systems generally use an Erbium Doped Fiber Amplifier (EDFA) to amplify the data light signals as required for long transmission distances. WDM systems using EDFA initially operated in the 1550 nm band which is the operating band of the Erbium Doped Fiber Amplifier and the band at which gain flattening can be easily achieved. While use of WDM communication systems using the EDFA has recently expanded to the small gain coefficient band of 1580 nm, there has nevertheless been an increasing interest in an optical amplifier that operates outside the EDFA band because the low loss band of an optical fiber is wider than a band that can be amplified by the EDFA; a Raman amplifier is one such optical amplifier.
In a Raman amplifier system, a strong pumping light beam is pumped into an optical transmission line carrying an optical data signal. As is known to one of ordinary skill in the art, a Raman scattering effect causes a gain for optical signals having a frequency approximately 13 THz smaller than the frequency of the pumping beam. Where the data signal on the optical transmission line has this longer wavelength, the data signal is amplified. Thus, unlike an EDFA where a gain wavelength band is determined by the energy level of an Erbium ion, a Raman amplifier has a gain wavelength band that is determined by a wavelength of the pumping beam and, therefore, can amplify an arbitrary wavelength band by selecting a pumping light wavelength. Consequently, light signals within the entire low loss band of an optical fiber can be amplified with the WDM communication system using the Raman amplifier and the number of channels of signal light beams can be increased as compared with the communication system using the EDFA.
For the EDFA and Raman amplifiers, it is desirable to have a high output laser device as a pumping source. This is particularly important for the Raman amplifier, which amplifies signals over a wide wavelength band, but has relatively small gain. Such high output is generally provided by a pumping source having multiple longitudinal modes of operation. The Furukawa Electric Co., Ltd. has recently developed an integrated diffraction grating device that provides a high output laser beam suitable for use as a pumping source in a Raman amplification system. An integrated diffraction grating device, as opposed to a fiber brag grating device, includes the diffraction grating formed within the semiconductor laser device itself. Examples of integrated diffraction grating devices are disclosed in U.S. patent application Ser. No. 09/832,885 filed Apr. 12, 2001, Ser. No. 09/983,175 filed on Oct. 23, 2001, and Ser. No. 09/983,249 filed on Oct. 23, 2001, assigned to The Furukawa Electric Co., Ltd. the entire contents of these applications are incorporated herein by reference. While the integrated diffraction grating devices disclosed in these applications provide an improved pumping source for optical amplifiers, the devices are manufactured to output a fixed wavelength. This limits the versatility of the integrated diffraction grating device.
An object of the present invention is to provide an integrated diffraction grating device having a tunable wavelength output.
Another object of the present invention is to provide a tunable laser device suitable for providing a pumping source to an optical amplifier such as a Raman amplifier.
According to a first aspect of the present invention, a semiconductor laser device and method for providing a light source suitable for use as a pumping light source in a Raman amplification system are provided. The laser device upon which the method is based includes a light reflecting facet positioned on a first side of the semiconductor device, a light emitting facet positioned on a second side of the semiconductor device thereby forming a resonator between the light reflecting facet and the light emitting facet, and an active layer configured to radiate light in the presence of an injection current, the active layer positioned within the resonator. A wavelength selection structure is positioned within the resonator and configured to select a spectrum of the light including multiple longitudinal modes, the spectrum being output from the light emitting facet. Also, an electrode positioned along the resonator and configured to provide the injection current, and a tuning current that adjusts a center wavelength of the spectrum selected by the wavelength selection structure.
The wavelength selection structure may be a diffraction grating positioned along a portion of the active layer in a distributed feedback (DFB) configuration, or positioned within a wavepath layer positioned along a portion of the resonator length where no active layer exists in a distributed Bragg reflector (DBR) configuration. In either configuration, the electrode of the may include a first a first portion configured to provide the injection current and positioned along the active layer where no diffraction grating or wavepath layer exists, and a second portion positioned along the diffraction grating or wavepath layer and configured to supply the tuning current to the diffraction grating. The first and second portions of the electrode may be electrically connected, or electrically insulated so as to be independently adjustable. Moreover, the active layer may be a quantum dot structure and the diffraction grating or wavepath layer may be positioned adjacent to either or both of the light emitting and light reflecting facets.
In yet another aspect of the present invention, a semiconductor laser module, optical fiber amplifier, Raman amplifier, and wavelength division multiplexing system are provided with the semiconductor laser device described above.